Negative commandment · Lo Ta’aseh #217 of 613
Not to crossbreed different species of animals
שלא להרביע בהמה מין בשאינו מינו
The verse · Vayikra 19:19
אֶת־חֻקֹּתַי תִּשְׁמֹרוּ בְּהֶמְתְּךָ לֹא־תַרְבִּיעַ כִּלְאַיִם שָׂדְךָ לֹא־תִזְרַע כִּלְאָיִם וּבֶגֶד כִּלְאַיִם שַׁעַטְנֵז לֹא יַעֲלֶה עָלֶיךָ
“You shall keep my statutes. Thou shalt not let thy cattle gender with a diverse kind: thou shalt not sow thy field with mingled seed: neither shall a garment mingled of linen and wool come upon thee.”
The Rambam’s words
Not to crossbreed different species of animals, as [Leviticus 19:19] states: "Do not crossbreed your livestock with other species."
שלא להרביע בהמה מין בשאינו מינו, שנאמר "בהמתך לא תרביע כלאיים" (ויקרא יט,יט).
Why · Sefer HaChinuch #244
To not mate a beast, a species with not its species
“It is from the roots of the commandment that G-d, blessed be He, created His world with wisdom, with understanding and with knowledge, and [so] He made and formed all of the forms according to that which it was fitting for its matter to be, to be designed [according to] the design of the world — and blessed is He that knows [this].”
Who & when
“And it is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Diverse Species הלכות כלאים